The invention relates generally to communication systems using low bandwidth wires such as telephone and Ethernet lines and more particularly to accurate reconstruction of a high frequency signal which had its frequency shifted to lower frequencies in order to enable its transmission over limited bandwidth media.
Several systems and application exist today where cellular or other high frequency signals need to be propagated through copper wires such as telephone or Ethernet lines. Since the bandwidth of copper wires is limited, high frequency signals can not propagate through them without significant attenuation. Therefore a known solution is to use a system in which the original high frequency signal is converted into a lower frequency band, which can be propagated over the copper wire and converted back to its original frequency band at the receiving side. Such a system is shown in FIG. 1. Frequency conversion is performed by mixing the incoming high frequency signal with a signal obtained by a local oscillator. The incoming high frequency signal at frequency band F1 is mixed by mixer 104 with the signal of a local oscillator 106 which its frequency is f2. The product of the mixer may be at a frequency described by F3=±F1±f2. A filter 108 at the output of the mixer 106 determines which of the possible combinations of F3 will be selected. As an example to be used in the following explanation, the required combination may be F3=f2−F1. The signal with this frequency is fed to the copper wire 109 and arrives through it to the remote unit which includes, among other elements a mixer 112, a local oscillator 114 and a band pass filter 118. The incoming signal at frequency band F3 is mixed at mixer 112 with a signal at frequency f5 produced by local oscillator 114. According to this example, filter 118 at the output of the mixer 112 selects a frequency band F6=f5−F3. In order for F6 to be an accurate replica of F1, f5 needs to be exactly the same frequency as f2. In other words, the frequency of both local oscillators needs to be the identical.
The method of transferring high frequency signals over low bandwidth media by frequency conversion is known in the art. However, this method requires the transfer of a reference signal between the master and the remote units in order to produce an accurate replica of the original signal at the remote side. U.S. Pat. No. 6,157,810 entitled “Distribution of Radio-Frequency Signals Through Low Bandwidth Infrastructure” to Georges describes such a system and proposes to transfer between the master unit and the remote unit a “reference tone” in a “IF frequency”, low enough to be transferred through the wires.
This approach, as used also in other systems, requires a dedicated special frequency band in the limited bandwidth of the wires and use of a relatively sharp band pass filter in the remote unit in order to filter the reference tone out of the other signals. This becomes even more problematic when several frequency bands need to be transferred simultaneously through the wires.